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Question gamma transmission -- physics models incorrect for thick materials  

Forum: Physics List
Date: 09 Jan, 2015
From: Chris Howard <Chris Howard>

I'm trying to reproduce a measurement and calculation using Geant4 to expand our design. At the basic level I have a point 60Co source located behind a concrete wall and I'm trying to measure the dose on the other side of the wall.

I've tried both the GPS 60Co source and a user defined 60Co source (created by me in PrimaryGeneratorAction). I have a 50x50x50 cm water cube on the other side of the concrete which is set as a scoring volume. The concrete is defined using the NIST database (G4_CONCRETE).

I've tried the physics models: 1) 'Shielding' model, 2) 'EmStandardPhysics_option3', and 3) 'EmStandardPhysics_option3+EmExtraPhysics', they all give the same results.

To determine the 'transmission' I take the ratio of the dose (E/mass) between runs with and without the wall present. I then do this for multiple thicknesses of the wall.

I get the attached results. After about 80 cm the MC results diverge from the calculation. The calculation takes 'build-up' (secondaries produced by the gammas from the decay) into account and has been validated by measurement.

Does anyone have any idea why Geant4 (10.1) would not be able to do this?

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2015/01/09/10.38-1759-MCvsCalc.png

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1 Idea: Re: gamma transmission -- physics models incorrect for thick materials   (Vladimir Ivanchenko - 11 Jan, 2015)
(_ Ok: Re: gamma transmission -- physics models incorrect for thick materials   (Chris Howard - 12 Jan, 2015)
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